The present invention relates generally to electronic circuitry and more particularly to isolation circuitry such as used in telephone line interface equipment.
Isolation of end user telephone equipment from the telephone network is required by the U.S. Federal Communications Commission and other counterpart regulatory agencies in other countries. The primary purpose of this regulation is to protect the network from faulty end user devices and from accidental connections to power lines and the like.
The specific regulations for such isolation change from country to country. The U.S. requires 1000 VRMS isolation while other countries may require 3500 to 4000 volt isolation. These regulations are usually dependent on power line mains voltage and internal country safety requirements. Common wisdom among those skilled in the field of Direct Access Arrangements or DAA's has shown other techniques than the application of isolation transformers between the end user equipment and the network. There has been some recent recognition that certain paths can be isolated using other techniques. For example, U.S. Pat. No. 4,864,605 to Ramsey et al shows the use of a coupling transformer in the main signal path and optical isolators in the ring detect and off-hook driving circuitry. In a similar way, U.S. Pat. No. 4,417,099 to Pierce discloses the use of optoisolators in the digital portion of the data paths of a line-powered modem. Moreover, U.S. Pat. No. 4,757,528 to Falater et al discloses an electro-thermal isolator coupler in conjunction with an RF capacitive coupling mechanism. This system claims to provide a DAA device, but requires a substrate having a subscriber portion and a line portion electrically isolated from each other.
Moreover, U.S. Pat. No. 5,245,654 to Wilkison et al discloses an optical isolation coupler circuit characterized by wide band width, low noise, and high linearity. This circuit has proven expensive due to the tightly coupled optical devices required to provide the necessary feedback for linearity.
The present invention eliminates the requirement for thermal, resistive, and optical isolation. The thermal and optical isolation requirements are complex and expensive to produce. The use of an integrated circuit to thermally transfer low frequency signals can be supplanted by other means that are more cost effective to produce. The capacitive approach lends itself to a more integrated design using less external components.